sh: Add support ESPT-GIGA borad

ESPT-Giga is SH7763-based reference board.
Board support is relatively sparse, presently supporting serial,
gigabit ethernet, USB host, and MTD.

More information (in Japanese) available at:
http://www.cente.jp/product/cente_hard/ESPT-Giga.html



Signed-off-by: Nobuhiro Iwamatsu <iwamatsu.nobuhiro@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>